Halogen flame retardants have been generally used in printed circuit boards to achieve flame retardancy. When halogen flame retardants combust, they emit heavy smoke and unpleasant odor, and produce hydrogen halide gas having a strong causticity. It is reported that, in recent years, halogen-containing flame retardants will produce carcinogen substances during pyrolysis and combustion, such as dioxins, dibenzofurans and the like. Waste Electrical and Electronic Equipment Directive and the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment were implemented by European Union on Jul. 1, 2006. The development of halogen-free flame retardant printed circuit boards has become a key point in the industry.
On the other aspect, the development of electronic components in the direction of light, thin, short, high performance and high function is accompanied by high frequency and high speed signal transmission. Besides, the delamination of high multi-layer PCB boards due to worse humidity and heat resistance and water absorption of CCL has attracted great attention.
DOPO is a common additive used in resin compositions, and is mainly used for increasing the flame retardancy of the resin composition and making the resin composition stable while being applied in high temperature environment.
Song Xiaoyun discloses synthesizing phosphorus-containing novolac resins by using DOPO and linear novolac (Song Xiaoyun, Synthesis and Application of Phosphorus-Containing Phenol Resin, Twelfth China CCL Market Symposium, 2009, p. 225).
CN103724997A discloses using polycyclic benzoxazine resins, multifunctional epoxy resins, and phosphorus-containing novolac resins which are linear phosphorus-containing novolac resins and have worse dielectric properties and water absorption although having a higher heat resistance.
CN 102585126B discloses reacting phenol with phosphorus oxychloride to synthesize a phosphorous-containing compound, then reacting with novolac resin to synthesize a phosphorous-containing novolac resin.
CN102206397B discloses using epoxy resins, hardening accelerators, crosslinking agents and phosphorous-containing resins which are bisphenol novolac resins replaced with 9,10-dihydro-9-oxa-10-phosphapheanthrene-10-oxide or derivatives thereof, to synthesize a copper clad laminate. Due to the presence of bisphenol structure in such phosphorous-containing novolac resin, it has worse humidity and heat resistance, high water absorption and worse dielectric properties.
CN 103382242A discloses synthesizing novolac resins by using phenol compounds, bisphenol compounds and formaldehyde, mixing novolac resins with aromatic phosphates to carry out a polycondensation and to obtain phosphorous-containing flame retardant novolac resins. The structures of bisphenol compounds and phenol compounds are used to improve the heat resistance of the current phosphorous-containing bisphenol novolac resins alone. However, the bisphenol compounds in the structure are bisphenol A/bisphenol F/bisphenol S and the like, and their heat resistance, dielectric properties and water absorption need to be further improved.
Therefore, it is the technical problem to be solved in the art how to have good dielectric properties and low water absorption, while having high heat resistance, and to achieve halogen-free flame retardancy.